Problem: Stephanie is 3 years older than Daniel. Thirteen years ago, Stephanie was 4 times as old as Daniel. How old is Daniel now?
Explanation: We can use the given information to write down two equations that describe the ages of Stephanie and Daniel. Let Stephanie's current age be $s$ and Daniel's current age be $d$ The information in the first sentence can be expressed in the following equation: $s = d + 3$ Thirteen years ago, Stephanie was $s - 13$ years old, and Daniel was $d - 13$ years old. The information in the second sentence can be expressed in the following equation: $s - 13 = 4(d - 13)$ Now we have two independent equations, and we can solve for our two unknowns. Because we are looking for $d$ , it might be easiest to use our first equation for $s$ and substitute it into our second equation. Our first equation is: $s = d + 3$ . Substituting this into our second equation, we get the equation: $(d + 3)$ $-$ $13 = 4(d - 13)$ which combines the information about $d$ from both of our original equations. Simplifying both sides of this equation, we get: $d - 10 = 4 d - 52$ Solving for $d$ , we get: $3 d = 42$ $d = 14$.